1. Field of the Disclosure
The present disclosure relates in general to using nuclear radiation measurements to measure formation characteristics in petroleum exploration. More particularly, the present disclosure provides a method for calibrating the effects of standoff on radiation measurements obtained with an apparatus conveyed in a borehole.
2. Description of the Related Art
Oil well logging has been known for many years and provides an oil and gas well driller with information about the particular earth formation being drilled. In conventional oil well logging, after a well has been drilled, a probe known as a sonde is lowered into the borehole and used to determine some characteristic of the formations which the well has traversed. The probe is typically a hermetically sealed steel cylinder which hangs at the end of a long cable which gives mechanical support to the sonde and provides power to the instrumentation inside the sonde. The cable also provides communication channels for sending information up to the surface. It thus becomes possible to measure some parameter of the earth's formations as a function of depth, that is, while the sonde is being pulled uphole. Such “wireline” measurements are normally done in real time (however, these measurements are taken long after the actual drilling has taken place).
Examples of prior art wireline density devices are disclosed in U.S. Pat. Nos. 3,202,822; 3,321,625; 3,846,631; 3,858,037; 3,864,569 and 4,628,202. Wireline formation evaluation tools have many drawbacks and disadvantages including loss of drilling time, the expense and delay involved in tripping the drillstring so as to enable the wireline to be lowered into the borehole and both the build up of a substantial mud cake and invasion of the formation by the drilling fluids during the time period between drilling and taking measurements. An improvement over these prior art techniques is the art of measurement-while-drilling (MWD) in which many of the characteristics of the formation are determined substantially contemporaneously with the drilling of the borehole.
Measurement-while-drilling logging either partly or totally eliminates the necessity of interrupting the drilling operation to remove the drillstring from the hole in order to make the necessary measurements by wireline techniques. In addition to the ability to log the characteristics of the formation through which the drill bit is passing, this information on a real time basis provides substantial safety advantages for the drilling operation.
Testing equipment conveyed downhole transmits energy into the formation from an energy source and performs measurements at a suitable receiver. These measurements may include resistivity, acoustic, or nuclear measurements. In nuclear radiation testing, the measurement tool generally comprises a gamma ray source and at least two detectors, such as NaI detectors, placed along the axis of the tool, such as a near detector placed proximate the gamma ray source and a far detector placed distal the gamma ray source. The gamma ray source and gamma ray detectors are shielded from each other to prevent counting of radiation emitted directly from the source. The gamma ray source emits nuclear energy, and more particularly gamma rays (high energy photons), and the corresponding detectors record the interaction of the gamma rays with the surrounding formation. The measurements derived from these interactions can be used to obtain a formation density. These interactions include photoelectric absorption, Compton scattering, or pair production.
Compton scattering is an interaction by which energy is transferred from the gamma ray to the electrons in the formation. This interaction forms the basis of the density measurement. Since the number of scattered gamma rays which reach the detectors is directly related to the number of electrons in the formation, the tool responds to the electron density of the rocks, which is in turn related to the bulk density. In the Compton scattering process, the involved photon loses some of its energy while changing its original direction of travel, the loss being a function of the scattering angle. Some of the photons emitted from the source into the sample are accordingly scattered toward the detector. Many of these never reach the detector, since their direction is changed by a second Compton scattering, or they are absorbed by the photoelectric absorption process or the pair production process. The scattered photons that reach the detector and interact with it are counted by the electronic equipment associated with the detector.
The photoelectric effect describes the case in which a gamma ray interacts with and transfers its energy to an atomic electron, ejecting that electron from the atom. The kinetic energy of the resulting photoelectron is equal to the energy of the incident gamma photon minus the binding energy of the electron. The photoelectric effect is the dominant energy transfer mechanism for low energy gamma rays, but it is much less important at higher energies. Photoelectric absorption often occurs when the gamma rays reach a low energy level after being repeatedly scattered by other electrons in the formation. A photoelectric effect factor can be determined by comparing the counts in a high energy region, where Compton scattering dominates, with those in a low energy region, where neither Compton scattering nor photoelectric effects dominate the other.
A reliable density measurement preferably maintains good contact between pad and formation. One potential problem with MWD logging tools is that there can be variations in the spacing between the logging tool and the borehole wall (“standoff”). Nuclear measurements are particularly degraded by large standoffs due to the scattering produced by borehole fluids in the annular region between the tool and the formation. The far detector generally is used because it has a greater depth of investigation. The response of the short-spacing detector, which is mostly influenced by drilling mud and borehole rugosity, can be used to correct the density measurement for these effects.
Because a caliper measurement is typically made during the testing, it is possible to check the quality of the contact. The presence of drilling mud and hole irregularities are usually accounted for using a “spine and ribs” chart based on a series of laboratory measurements. A spine and ribs correction technique is well known in the nuclear well logging art of density logging. Such correction technique is based on a well known correction curve by Wahl, J. S., Tittman, J., Johnstone, C. W., and Alger, R. P., “The Dual Spacing Formation Density Log”, presented at the Thirty-ninth SPE Annual Meeting, 1964. Such curve includes a “spine” which is a substantially linear curve relating the logarithm of long spacing detector count rates to the logarithm of short spacing detector count rates. Such curve is marked by density as a parameter along the curve. “Ribs” cross the spine at different intervals. Such ribs are experimentally-derived curves showing the correction necessary for different mudcake conditions. The short and long spacing readings are automatically plotted on this chart and corrected for their departure from true value.
Accounting for the standoff between logging tool and formation is an important aspect of obtaining accurate radiation measurements of formation properties. The present disclosure provides a method of calibrating a density measurement for standoff effects using a single detector spectrum without using caliper measurements.